The Dark Energy Survey looks at massive galaxy clusters – and finds filaments

With data from the new DECam imager, the Dark Energy Survey provides additional clues that galaxy clusters are not isolated objects on the sky but are connected with the cosmic web via filaments. Shown here is the famous Bullet cluster. Image: Dark Energy Survey

Galaxy clusters — accumulations of hundreds of galaxies — are said to be the largest gravitationally bound structures in the universe. While this statement is correct as such, it easily conveys an incorrect picture: that of clusters as static, isolated spheres that have swallowed every galaxy within reach at some time in the cosmic past. Nothing could be further from reality.

Galaxy clusters are not isolated but dynamic environments that actively accrete material from their surroundings. The preferred mode of accretion proceeds along so-called filaments, the connecting links between the central hubs of the cosmic web. The existence of filaments is a prediction of the cold dark matter model we use to describe the formation of structures in the universe, revealed in large cosmological simulations and spectroscopic surveys.

The new Dark Energy Camera was built by the 300-member Dark Energy Survey (DES) collaboration to carry out a five-year survey to probe the origin of cosmic acceleration. The camera is mounted on the Blanco 4-meter telescope at the Cerro Tololo Inter-American Observatory in Chile and saw first light in September 2012.

Shortly after the camera was commissioned, we proposed a program to target several massive galaxy clusters as part of a process called science verification, a rigorous test of the new instrument. The prospects for this project were mixed. After the overhaul of the telescope control system and with the new camera, nobody could guarantee that the images we were going to obtain would have the necessary quality for accurate studies of these clusters. But if it worked, we could exploit DECam’s massive field of view of more than 3 square degrees (roughly 15 times the area of the full moon) to study not only the clusters themselves, but also the environments from which they accrete.

It worked. Over the course of 18 months, I led a team that ultimately involved more than 90 DES scientists from 37 institutions worldwide. In our recently submitted paper, the first based upon DES data, we demonstrated that the new camera and revamped telescope worked together as expected. This data and our careful analysis allowed us to determine the distributions of so-called red-sequence galaxies, whose red color is a reliable tracer of the dynamical processes in clusters. Furthermore, we exploited an effect called gravitational lensing to infer the mass distributions of these clusters, an analysis with exceptionally stringent requirements on image quality.

Everything lines up. The visible orientation of the brightest cluster galaxies sitting at the cluster centers; the mass distribution tracing hundreds of cluster galaxies (shown in the image below); the large-scale distribution of red-sequence galaxies far beyond the gravitational reach of the actual clusters: All these probes show that clusters are indeed interwoven with the cosmic web, the structure of which DES will reveal in unprecedented detail.

Peter Melchior, Ohio State University

This plot shows the mass distribution from weak-lensing measurements (contours) and red-sequence galaxies (black dots) of the galaxy cluster RXC J2248.7-4431 at redshift z=0.348. The red-sequence galaxy distribution extends substantially farther out, reaching a total length of approximately 1 degree, or twice the diameter of the full moon.
These scientists are the primary authors of this work. Top row, from left: Peter Melchior (Ohio State), Eric Suchyta (Ohio State), Eric Huff (Ohio State), Michael Hirsch (U College London). Bottom row, from left: Tomasz Kacprzak (Manchester), Eli Rykoff (SLAC), Daniel Gruen (University Observatory and MPE Munich).